The invention concerns a capillary metering device for a powder product comprising a vibration body with a capillary formed within the vibration body for receiving and conveying a powder product; the invention also concerns a method for metering a powder product by means of such a capillary metering device.
Fine-grain powder products, in particular medicaments in powder form, food supplements and the like, must be precisely metered in order to achieve a certain precisely measured administration dosage. Such precisely metered dosage quantities are, for example, filled into hard gelatin capsules or are pressed to tablets.
Frequently, for metering the powder product so-called capillary metering devices are employed. They comprise a vibration body with a capillary that is formed in the vibration body for receiving and conveying the powder product. The cross-section of the substantially vertically oriented capillary is sized and adapted to the material properties of the powder product so that the powder, as a result of its cohesive properties, cannot pass through the stationary capillary but remains adhered thereto. Inasmuch as the vibration body is however excited to perform vibrations with a suitable frequency and amplitude, the cohesive forces are overcome. As a result, a powder flow of the powder product through the capillary that is formed within the vibration body is generated and is guided into a target container. By switching on and switching off the vibration of the vibration body in a targeted fashion, the powder flow can be likewise switched on or switched off in a targeted fashion in order to obtain a certain dosage quantity of the powder product in the target container. In order for this type of metering to function properly in practice, a precise adaptation of the capillary cross-section to the powder properties, in particular the grain size distribution, grain shape, density and composition of the powder to be metered, is required. When too small a cross-section is selected, a powder flow cannot be generated even with vibration excitation of the vibration body. When too large a cross-section is selected, the cohesion forces of the powder inter alia may not be sufficient to stop the powder flow when the vibration excitation is switched off, or to inhibit its generation. For a proper adjustment, flow rates of approximately 1 mg per second up to 10 mg per second can be obtained according to the prior art. However, there is the need for a greater achievable range of flow rates.
For forming the aforementioned capillary, tubes that are comparable to the cannula of an injection syringe are employed according to the prior art. The tubes and the capillaries formed therein in have a circular cross-section that determines decisively the adhesion and flow properties of the powder. Clearly defined adhesion and flow properties can however only be found when the powder itself has a homogenous configuration of its grains. This is not always the case in practice. Individual larger powder grains or local agglomerate formations can lead to clogging of the capillary which impairs the continuous metering operation. Cleaning in case of clogging or a product change is possible only with difficulty because of the minimal capillary cross-section. Moreover, it has been found that even when selecting high-quality tubes the capillary walls have a significant and non-uniform surface roughness that causes flow and adhesion properties that are difficult to predict. A smoothing surface treatment or a suitable surface coating are hardly realizable because of the minimal capillary cross-section so that undesirable fluctuations of the adhesion and flow properties are unavoidable.
The invention has the object to further develop a capillary metering device in such a way that a precise metering with increased reliability is possible.